1. Field of the Invention
The present invention relates to communication data links of the type employed between an airborne platform or platforms and ground stations. More particularly, the present invention relates to a novel multi-channel data link where each channel contains a time of day spread spectrum waveform having means for independently advancing and retarding the time of day in each of the quadrature channels to enable instantaneous reacquisition of a ground station after an outage has occurred.
2. Description of the Prior Art
It is known that airborne surveillance platforms are used to capture optical images, infrared images and/or radar images and to process the sensed image information into a digital format and then communicate this image information to plural ground stations over a data link using secure spread spectrum modulated waveforms. When the airborne platform encounters an obstacle between it and the ground station, the ground station loses its spread spectrum synchronization or lock of the down link signal. Correspondingly, the airborne platform also loses its synchronization or lock on the up link signal from the ground station. Any attempt by the airborne platform to advance or retard its down link signal to aid the ground station to reacquire the down link signal will cause the other ground stations to lose their lock or synchronization on their down link signals.
The problem is made worse when the down link signals are transmitted in predetermined time slots using Time Division Multiple Access (TDMA) format because the airborne platform is moving and its signal time of propagation is continuously changing.
Heretofore, multi-channel in-phase (I) and quadrature phase (Q) multi-channel transmitters were known, but were employed to enhance the amount of power received at a receiver. Such systems are explained in my U.S. Pat. No. 5,559,788 which is incorporated by reference herein. In the two co-pending U.S. applications referenced above, the power and data rate of the in-phase and quadrature channels were adjusted to overcome jamming of one ground station which could cause an outage of the network system or a reduction in the data rate of the entire network system.
None of the systems discussed hereinbefore have addressed the problem that occurs when an outage of one or more ground stations in a network system requires reacquisition of the ground station as rapidly as possible. Thus, it would be desirable to provide an apparatus and a method for instantaneously reacquiring a ground station back into a network system after an outage has occurred.
It is a principal object of the present invention to virtually eliminate reacquisition time of a ground station into a surveillance system or network.
It is a principal object of the present invention to provide a multi-channel Data Link System in which one quadrature channel is maintained as a master time of day spread spectrum waveform and the other channel is independently varied in phase time relationship relative to the master channel to provide a reacquisition down link signal to the ground station.
It is a principal object of the present invention to calculate a propagation time from an airborne station or node to a ground station at the time of reacquisition so that the down link signal is synchronized at the time of reception by the ground station""s receiver whose time of day clock is known.
It is a principal-object of the present invention to calculate a time of day correction signal at the air-borne station for a ground station to enable the ground station to reacquire and resynchronize its time of day clock with the master time of day clock in the airborne station.
It is a principal object of the present invention to resynchronize the phase and time of day signal of the adjustable in-phase channel in the airborne station with the master channel once all ground stations have been reacquired.
According to these and other objects of the present invention there is provided in a Data Link Network System a central node or airborne station with a multi-channel transmitter/receiver capable of transmitting quadrature phase spread spectrum signals both of which contain data. One channel is maintained as a master channel which provides a master time of day clock for the network system and the other channel is employed as an adjustable time of day channel. After an outage between the airborne station and one of its ground station""s the airborne station calculates the propagation time to the ground station and shifts the adjustable channel to transmit a spread spectrum data signal which will arrive in synchronization with the time of day clock in the ground station that has the outage. The transmitted data signal is instantaneously reacquired by the ground station and the data accompanying the down link transmission is employed to resynchronize the ground station receiver/transmitter with the master clock in the airborne receiver/transmitter.